Dengue serotype 1 attenuated strain

ABSTRACT

The invention relates to live attenuated VDV1 (VERO-Derived Dengue serotype 1 virus) strains which have been derived from the wild-type dengue-1 strain 16007 by passaging on PDK and sanitization on Vero cells. The invention further relates to a vaccine composition which comprises a VDV1 strain.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. provisional application 60/691,243, filed on Jun. 17, 2005, incorporated herein by reference.

The invention relates to new live attenuated VDV1 (VERO-Derived Dengue serotype 1 virus) strains which are derived from the wild-type dengue-1 strain 16007 by passaging on PDK and Vero cells, and sanitization. The invention further relates to a vaccine composition which comprises such VDV1 strain.

Dengue diseases are caused by four closely related, but antigenically distinct, virus serologic types (Gubler, 1988; Kautner et al., 1997; Rigau-Pérez et al., 1998; Vaughn et al., 1997), of the genus Flavivirus (Gubler, 1988). Infection with a dengue virus serotype can produce a spectrum of clinical illnesses ranging from a non-specific viral syndrome to severe, fatal haemorrhagic disease. The incubation period of dengue fever (DF) after the mosquito bite averages 4 days (range 3-14 days). DF is characterised by biphasic fever, headache, pain in various parts of the body, prostration, rash, lymphadenopathy and leukopenia (Kautner et al., 1997; Rigau-Pérez et al., 1998). The viremic period is the same as of febrile illness (Vaughn et al., 1997). Recovery from DF is usually complete in 7 to 10 days but prolonged asthenia is common. Leukocytes and platelets counts decreases are frequent.

Dengue haemorrhagic fever (DHF) is a severe febrile disease characterised by abnormalities of homeostasis and increased vascular permeability that can lead to hypovolemia and hypotension (dengue shock syndrome, DSS) often complicated by severe internal bleeding. The case fatality rate of DHF can be as high as 10% without therapy, but below 1% in most centres with therapeutic experience (WHO Technical Guide, 1986).

Routine laboratory diagnosis of dengue infections are based on virus isolation and/or the detection of dengue virus-specific antibodies.

Dengue disease is the second most important tropical infectious disease after malaria, with over half of the world's population (2.5 billion) living in areas at risk for epidemic transmission. An estimated 50 to 100 million cases of dengue, 500,000 hospitalized DHF patients and 25,000 deaths occur each year. Dengue is endemic in Asia, the Pacific, Africa, Latin America, and the Caribbean. More than 100 tropical countries have endemic dengue virus infections, and DHF have been documented in more than 60 of these (Gubler, 2002; Monath, 1994). A number of well described factors appear to be involved in dengue infections: population growth, unplanned and uncontrolled urbanization particularly in association with poverty, increased air travel, lack of effective mosquito control, and the deterioration of sanitary and public health infrastructure (Gubler, 2002). The awareness of dengue in travelers and expatriates is increasing (Shirtcliffe et al., 1998). Dengue has proven to be a major cause of febrile illness among US troops during deployments in dengue-endemic tropical areas (DeFraites et al., 1994).

The viruses are maintained in a cycle that involves humans and Aedes aegypti, a domestic, day-biting mosquito that prefers to feed on humans. Human infection is initiated by the injection of virus during blood feeding by an infected Aedes aegypti mosquito. Salivary virus is deposited mainly in the extravascular tissues. The primary cell subset infected after inoculation is dendritic cells, which subsequently migrate to draining lymph nodes (Wu et al., 2000). After initial replication in the skin and draining lymph nodes, virus appears in the blood during the acute febrile phase, generally for 3 to 5 days.

Monocytes and macrophages are with dendritic cells among the primary target of dengue virus. Protection against homotypic reinfection is complete and probably lifelong, but cross-protection between dengue types lasts less than 12 weeks (Sabin, 1952). Consequently a subject can experience a second infection with a different serotype. A second dengue infection is a theoretical risk factor of developing severe dengue disease. However, DHF is multifactorial including: the strain of the virus involved, as well as the age, immune status, and genetic predisposition of the patient. Two factors play a major role in the occurrence of DHF: a rapid viral replication with high viremia (the severity of the disease being related to the level of viremia (Vaughn et al., 2000) and an important inflammatory response with release of high levels of inflammatory mediators (Rothman and Ennis, 1999).

There is no specific treatment against Dengue diseases. The management of DF is supportive with bed rest, control of fever and pain with antipyretics and analgesics, and adequate fluid intake. The treatment of DHF needs correction of fluid loss, replacement of coagulation factors, and infusion of heparin.

Preventive measures presently rely on vector control and personal protection measures, which are difficult to enforce and expensive. No vaccine against dengue is currently registered. Since the 4 serotypes of dengue are circulating worldwide and since they are reported to be involved in cases of DHF, vaccination should ideally confer protection against all 4 dengue virus serotypes.

Live attenuated vaccines (LAVs), which reproduce natural immunity, have been used for the development of vaccines against many diseases, including some viruses belonging to the same genus as dengue (examples of commercially available flavivirus live-attenuated vaccines include yellow fever and Japanese encephalitis vaccines). The advantages of live-attenuated virus vaccines are their capacity of replication and induction of both humoral and cellular immune responses. In addition, the immune response induced by a whole virion vaccine against the different components of the virus (structural and non-structural proteins) reproduced those induced by natural infection.

A dengue vaccine project was initiated in Thailand at the Centre for Vaccine Development, Institute of Sciences and Technology for Development Mahidol University. Candidate live-attenuated vaccines were successfully developed, at a laboratory scale, for dengue serotype 1 (strain 16007, passage 13=LAV1), serotype 2 (strain 16681, passage 53), and serotype 4 (strain 1036, passage 48) viruses in Primary Dog Kidney (PDK) Cells, and for serotype 3 (strain 16562) in Primary Green Monkey Kidney (PGMK) cells (passage 30) and Fetal Rhesus Lung (FRhL) cells (passage 3). These vaccines have been tested as monovalent (single serotype), bivalent (two serotypes), trivalent (three serotypes), and tetravalent (all four serotypes) vaccines in Thai volunteers. Those vaccines were found to be safe and immunogenic in children and in adults (Gubler, 1997). These LAV 1-4 strains have been described in EP 1159968 in the name of the Mahidol University and were deposited before the CNCM (CNCM I-2480; CNCM I-2481; CNCM I-2482 and CNCM I-2483 respectively).

The complete sequence of the Dengue 1 Live-Attenuated Virus strain (LAV1) was established by R. Kinney et al. (CDC, Fort Collins). Sequence differences between parent DEN-1 strain 16007 (SEQ ID No.2) and LAV1 (SEQ ID No.3) strain are described in Table 1. Thus, genetic comparison of the wild-type virus strain 16007 and LAV1 strain showed a set of 14 point mutations which could be linked to LAV1 attenuation.

TABLE 1 DEN-1 16007 and DEN-1 16007/PDK13 (LAV1) Sequence Differences LAV1 (DEN-1 16007/ Coordinates PDK13) 16007 Gene-aa position Nt aa nt aa E-130 Nt-1323 C Ala T Val E-203 Nt-1541 A Lys G Glu Nt-1543 G A E-204 Nt-1545 A Lys G Arg E-211 Nt-1567 G Gln A Gln E-225 Nt-1608 T Leu C Ser E-477 Nt-2363 G Val A Met NS1-92 Nt-2695 C Asp T Asp NS1-121 Nt-2782 T Ala C Ala NS3-182 Nt-5063 A Lys G Glu NS3-510 Nt-6048 T Phe A Tyr NS4A-144 Nt-6806 G Val A Met NS4B-168 Nt-7330 G Gln A Gln NS5-624 Nt-9445 T Ser C Ser Nucleotide changes modifying the corresponding codon are indicated in bold.

The LAV1 strain which was initially established in 1983 was further rapidly identified as potential vaccine candidate (Bhamarapravati and Yoksan, 1997).

However, at that time, transmission to humans of Spongiform Encephalitis through mammalian cultures was not perceived as a risk and the virus was routinely maintained in Primary Dog Kidney cells (PDK). Furthermore, this LAV1 strain corresponds to a heterogeneous population. This heterogeneity represents an additional risk due to a potential in vitro or in vivo selection of one of the strain present in the composition.

In view of these increasing concerns, the Applicant decided to set up a sanitization process in order to get rid of any such risks. By first transferring the LAV1 vaccine strain from PDK to VERO cells and then transfecting Vero cells with the purified genomic RNA of LAV1, followed by two successive steps of virus plaque purification the Applicant produced a new Vero-Derived serotype 1 virus (VDV1).

This new VDV1 strain which has been thus derived by transfer to VERO cells and biological cloning differs from the LAV1 strain by sequence, an homogenous plaque size and temperature sensitivity but importantly has conserved some phenotypic and genotypic features of the LAV1 such as e.g. attenuation spots, small plaque phenotype, growth restriction at high temperature, and has conserved the immunogenic features of the LAV1 strains. These features make this new strain a valuable vaccine candidate for prophylactic immunization in humans.

DEFINITIONS

“Dengue viruses” are positive-sense, single-stranded RNA viruses belonging to the Flavivirus genus of the Flaviviridae family. In the case of dengue serotype 1 (DEN-1) strain 16007, the entire sequence is 10735 nucleotides long (SEQ ID No.2). The RNA genome contains a type I cap at the 5′-end but lacks a 3′-end poly (A)-tail. The gene organization is 5′-noncoding region (NCR), structural protein (capsid (C), premembrane/membrane (prM/M), envelope (E)) and non structural protein (NS1-NS2A-NS2B-NS3-NS4A-NS4B-NS5) and 3′NCR. The viral RNA genome is associated with the C proteins to form nucleocapsid (icosahedral symmetry). As with other flaviviruses, the DEN viral genome encodes the uninterrupted open reading frame (ORF) which is translated to a single polyprotein.

Serial passaging of a virulent (disease-causing) strain of dengue-1 results in the isolation of modified virus which are “live attenuated”, i.e., infectious, yet not capable of causing disease. These modified viruses are usually tested in monkeys to evaluate their attenuation. However, Humans are the only primates that exhibit signs of clinical disease. The viruses that cause mild (i.e. acceptable in terms of regulatory purposes as presenting a positive benefit/risk ratio) to low or no secondary effects (i.e. systemic events and/or biological abnormalities and/or local reactions) in the majority of the tested humans but still infect and induce an immune response are called “live attenuated”.

The term “LAV” denotes live attenuated Dengue viral strains. In the context of the invention “LAVs” are live attenuated strains initially derived from the Dengue serotype 1 (DEN-1) strain 16007 by passages, e.g. 10, 11, 12 or 13 passages, in Primary Dog Kidney (PDK) Cells. For instance “LAV1/PDK13” is the attenuated strain established after 13 passages of strain 16007 in PDK cells (also named DEN-1 16007/PDK13). LAV1/PDK13 nucleotide sequence is shown in SEQ ID No.3.

“VDV1” is meant a LAV obtainable by the sanitization process disclosed in the present application. A VDV1 is thus a biological clone (homogeneous) VERO-adapted Dengue serotype 1 virus capable of inducing a specific humoral immune response including neutralizing antibodies in primate especially in humans. The VDV1 strains of the invention can be easily reconstructed starting directly from the here disclosed VDV1 sequences. The induction of a specific humoral immune response can be easily determined by an ELISA assay. The presence of neutralizing antibody in the serum of a vaccinee is evaluated by the plaque reduction neutralization test as described in section 4.1.2.2. A serum is considered to be positive for the presence of neutralizing antibodies when the neutralizing antibody titer thus determined is at least superior or equal to 1:10.

The terms “mutation” means any detectable change in genetic material, e.g. DNA, RNA, cDNA, or any process, mechanism, or result of such a change. Mutations include substitutions of one or more nucleotides. In the context of the instant application, mutations identified in dengue-1 virus genomic sequence or polyprotein are designated pursuant to the nomenclature of Dunnen and Antonarakis (2000). As defined by Dunnen and Antonarakis at the nucleic acid level, substitutions are designated by “>”, e.g. “31A>G” denotes that at nucleotide 31 of the reference sequence a A is changed to a G.

Variations at the protein level describe the consequence of the mutation and are reported as follows. Stop codons are designated by X (e.g. R97X denotes a change of Arg96 to a termination codon). Amino acid substitutions are designated for instant by “S9G”, which means that Ser in position 9 is replaced by Gly.

VERO-Derived Dengue Serotype 1 Viruses (VDV1)

The composition of the previously developed dengue-1 vaccine candidate LAV1 was improved by a sanitization process.

The VERO-Derived Dengue serotype 1 viruses (VDV1) disclosed herein use the DEN-1 16007 virus attenuated by serial passages on PDK cells. VDV1 contains the whole genomic sequence of the live-attenuated DEN-1 virus, and bears the same spots which have been linked to attenuation as the original LAV1 strain that was tested in humans.

Sanitization of the LAV1 vaccine candidate was performed by removing proteins and introducing only purified viral genomic material into Vero cells. More specifically, sanitization of the strain was performed in 2 steps:

1) Amplification of DEN16007/PDK11 (LAV1/PDK11) on Vero cells, at 32° C.

2) Purification and transfection of viral RNA into Vero cells.

Step 1 has been carried out by one passage of LAV1/PDK11 on Vero cells. For that purpose, Vero cells were infected with LAV1/PDK11 at a moi of 0.01 and incubated at 32° C. for 5 days.

For step 2, advantage was taken of the fact that the viral genome is an infectious RNA, which means that it is able, when introduced into a cell, to reconstitute a complete infectious virus. The second purification and transfection step thus comprised the steps consisting of:

a) extracting and purifying viral RNA from plaque-purified viruses;

b) advantageously associating of the purified RNA with cationic lipids;

c) transfecting Vero cell, in particular Vero cell LS10;

d) recovering of the neo-synthesized viruses; and

e) purifying a VDV strain by plaque purification and optionally amplifying it in host cells, especially Vero cells.

The Vero cell technology is a well-known technology which has been used for different commercial products (injectable and oral polio vaccines, rabies vaccine). In the present invention qualified Vero cells were advantageously used to guarantee the absence of any risks potentially linked to the presence of adventitious agents. By “qualified VERO cells” is meant cells or cell lines for which culture conditions are known and is such that the said cells are free from any adventitious agents. These include e.g. the VERO cell LS10 of Sanofi Pasteur.

The thus isolated VDV strains are classically stored either in the form of a freezed composition or in the form of a lyophilised product. For that purpose, the VDV can be mixed with a diluent classically a buffered aqueous solution comprising cryoprotective compounds such a sugar alcohol and stabilizer. The pH before freezing or lyophilisation is advantageously settled in the range of 6 to 9, e.g. around 7 such as a pH of 7.5+/−0.2 as determined by a pH meter at RT. Before use, the lyophilised product is mixed with a pharmaceutically diluent or excipient such as a sterile NaCl 4% solution to reconstitute a liquid immunogenic composition or vaccine.

Sequencing, at attenuation-specific loci, of virus recovered after transfection, did not reveal any mutation, compared to the LAV1/PDK13 strain. The biologically cloned VDV1 virus exhibits a homogenous plaque phenotype and a remarkable genetic stability with regard to its LAV1 parent as it can be deduced especially from the conservation of the attenuation genotype.

VDV1 strain was sequenced and compared with the serotype 1 Dengue Live Attenuated Virus (LAV1/PDK13) strain sequence (SEQ ID No 3). A set of 3 nucleotide differences was found with regard to the reference LAV1 sequence. One of them is silent at the amino acid level (position 2719). The two others (positions 5962 and 7947) are located in non-structural peptides coding sequences (NS3-481 and NS5-125, respectively). None of these differences corresponds to any of the LAV1 attenuation positions.

The invention thus provides for live attenuated dengue-1 virus strains that have been obtained from the wild type virus DEN-1 16007 attenuated by serial passages on PDK cells and then by passage and sanitization on VERO cells. In particular the attenuated strains of the invention comprise at least the identified sequence mutations (non-silent and optionally silent) relative to the nucleotide sequence or polyprotein sequence of the wild-type DEN-1 16007 and LAV1/PDK13 strains.

Accordingly, the invention relates to an isolated live attenuated dengue-1 virus strain which comprises, or consists of, the sequence of LAV1/PDK13 strain (SEQ ID No.3) wherein at least nucleotides at positions 5962 and 7947, and optionally 2719, are mutated, with the proviso that the following nucleotides are not mutated: 1323, 1541, 1543, 1545, 1567, 1608, 2363, 2695, 2782, 5063, 6048, 6806, 7330, and 9445. Preferably, the mutations are substitutions. Preferably, the nucleotide at position 5962 is A, the nucleotide at position 7947 is G. Still preferably, the isolated strain according to the invention contains sequence SEQ ID No.3 which comprises the mutations 2719 G>A, 5962 C>A, and 7947 A>G.

Hence, a live attenuated dengue-1 virus strain according to the invention comprises the sequence of wild-type dengue-1 strain 16007 (SEQ ID No.2) wherein said sequence comprises at least the mutations 1323 T>C, 1541 G>A, 1543 A>G, 1545 G>A, 1567 A>G, 1608 C>T, 2363 A>G, 2695 T>C, 2782 C>T, 5063 G>A, 5962 C>A, 6048 A>T, 6806 A>G, 7330 A>G, 7947 A>G, and 9445 C>T. Preferably, a live attenuated strain according to the invention further comprises the mutation 2719 G>A by reference to the nucleotide sequence of wild-type strain 16007 (SEQ ID No.2).

The live attenuated dengue-1 virus strains according to the invention encompass variant strains that comprise a sequence SEQ ID No.3 mutated in positions 5962 and 7947, as defined above, and that further comprise a substitution of one or more nucleotides in a given codon position that results in no alteration in the amino acid encoded at that position.

Advantageously, the live attenuated dengue-1 virus strain according to the invention comprises a sequence which differs by a limited number of mutations, e.g. no more than 5, still preferably no more than 2, from SEQ ID No.1.

Preferably, the genomic sequence of a dengue-1 virus strain according to the invention consists of the nucleotide sequence SEQ ID No.1.

The invention also relates to live attenuated dengue-1 strains that may be derived from the VDV1 strain of sequence SEQ ID No.1 by further passages on cells, in particular Vero cells.

The invention also relates to an isolated nucleic acid which comprises, or consists of, the DNA sequence SEQ ID No.1 or its equivalent RNA sequence.

A “nucleic acid molecule” refers to the phosphate ester polymeric form of ribonucleosides (adenosine, guanosine, uridine or cytidine; “RNA molecules”) or deoxyribonucleosides (deoxyadenosine, deoxyguanosine, deoxythymidine, or deoxycytidine; “DNA molecules”), or any phosphoester analogs thereof, such as phosphorothioates and thioesters, in either single stranded form, or a double-stranded helix.

As used herein, by RNA sequence “equivalent” to SEQ ID No.1 is meant a sequence SEQ ID No.1 wherein deoxythymidines have been replaced by uridines. As SEQ ID No.1 constitutes VDV1 cDNA sequence, the equivalent RNA sequence thus corresponds to the positive strand RNA of VDV1.

The invention further relates to the polyprotein of sequence SEQ ID No.41 and to fragments thereof. SEQ ID No.41 is the sequence of the polyprotein encoded by SEQ ID No.1

A “fragment” of a reference protein is meant a polypeptide which sequence comprises a chain of consecutive amino acids of the reference protein. A fragment may be at least 8, at least 12, at least 20, amino acid long.

Said fragments of the polyprotein of sequence SEQ ID No.41 comprise at least a lysine at position 481 of NS3 protein (position 1956 of SEQ ID No.41), and/or an arginine at position 125 of NS5 protein (position 2618 of SEQ ID No.41).

According to an embodiment the fragment of the polyprotein encoded by SEQ ID No.1 is or comprises NS3 protein and/or NS5 protein.

Immunogenic and Vaccine Compositions

The invention also relates to an immunogenic composition, suitable to be used as a vaccine, which comprises a VDV1 strain according to the invention.

The immunogenic compositions according to the invention elicit a specific humoral immune response toward the dengue virus comprising neutralizing antibodies.

Preferably, the immunogenic composition is a vaccine.

According to an embodiment, the immunogenic is a monovalent composition, i.e. it elicits a specific immune response and/or confers protection against Dengue-1 virus only.

According to another embodiment, the invention relates to a multivalent dengue immunogenic composition. Such a multivalent immunogenic composition or vaccine may be obtained by combining individual monovalent dengue vaccines. The immunogenic or vaccine composition may further comprise at least a live attenuated dengue virus of another serotype. In particular, the immunogenic or vaccine composition may comprise a VDV1 according to the invention in combination with at least a live attenuated dengue virus selected from the group consisting of serotype 2, serotype 3, and serotype 4.

Preferably, the immunogenic or vaccine composition may be a tetravalent dengue vaccine composition, i.e. a vaccine composition that comprises a VDV1 according to the invention in combination with a live attenuated dengue-2 virus strain, a live attenuated dengue-3 virus strain and a live attenuated dengue-4 virus strain.

Live attenuated dengue-2, dengue-3 and dengue-4 virus strains have been described previously. Reference may be made to the live-attenuated vaccines that were developed by Mahidol University by passaging dengue serotype 2 (strain 16681, passage 53; LAV2), and serotype 4 (strain 1036, passage 48, LAV4) viruses in Primary Dog Kidney (PDK) Cells, and for serotype 3 (strain 16562) in Primary Green Monkey Kidney (PGMK) cells (passage 30) and Fetal Rhesus Lung (FRhL) cells (passage 3) (LAV3). The nucleotide sequences of LAV2 (SEQ ID No.42), LAV3 (SEQ ID No.43), and LAV4 (SEQ ID No.44) are shown in the annexed sequence listing.

Advantageously, a live attenuated dengue-2 strain may correspond to a VDV2 strain which has been obtained from the LAV2 strain developed by Mahidol by a process of sanitization on Vero cells. In particular a live attenuated dengue-2 strain (VDV2) may comprise, and advantageously consists of the sequence SEQ ID No.40.

Immunogenic compositions including vaccines may be prepared as injectables which can correspond to liquid solutions, suspensions or emulsions. The active immunogenic ingredients may be mixed with pharmaceutically acceptable excipients which are compatible therewith.

The immunogenic compositions or vaccines according to the present invention may be prepared using any conventional method known to those skilled in the art. Conventionally the antigens according to the invention are mixed with a pharmaceutically acceptable diluent or excipient, such as water or phosphate buffered saline solution, wetting agents, fillers, emulsifier stabilizer. The excipient or diluent will be selected as a function of the pharmaceutical form chosen, of the method and route of administration and also of pharmaceutical practice. Suitable excipients or diluents and also the requirements in terms of pharmaceutical formulation, are described in Remington's Pharmaceutical Sciences, which represents a reference book in this field.

Preferably, the immunogenic composition or vaccine corresponds to an injectable composition comprising an aqueous buffered solution to maintain e.g. a pH (as determined at RT with a pH meter) in the range of 6 to 9.

The composition according to the invention may further comprise an adjuvant, i.e. a substance which improves, or enhances, the immune response elicited by the VDV1 strain. Any pharmaceutically acceptable adjuvant or mixture of adjuvants conventionally used in the field of human vaccines may be used for this purpose.

The immunogenic compositions or vaccines according to the invention may be administered by any conventional route usually used in the field of human vaccines, such as the parenteral (e.g. intradermal, subcutaneous, intramuscular) route In the context of the present invention immunogenic compositions or vaccines are preferably injectable compositions administered subcutaneously in the deltoid region.

Method for Immunizing

The invention further provides for a method of immunizing a host in need thereof against a dengue infection which comprises administering the host with an immunoeffective amount of an immunogenic composition or a vaccine according to the invention.

A “host in need thereof” denotes a person at risk for dengue infection, i.e. individuals travelling to regions where dengue virus infection is present, and also inhabitants of those regions.

The route of administration is any conventional route used in the vaccine field. The choice of administration route depends on the formulation that is selected. Preferably, the immunogenic composition or vaccine corresponds to an injectable composition administered via subcutaneous route, advantageously in the deltoid region.

The amount of LAV or VDV in particular VDV1 in the immunogenic compositions or vaccines may be conveniently expressed in viral plaque forming unit (PFU) unit or Cell Culture Infectious Dose 50% (CCID₅₀) dosage form and prepared by using conventional pharmaceutical techniques. For instance, the composition according to the invention may be prepared in dosage form containing 10 to 10⁶ CCID₅₀, or from 10³ to 10⁵ CCID₅₀ of LAV or VDV, for instance 4±0.5 log₁₀ CCID₅₀ of VDV1 strain for a monovalent composition. Where the composition is multivalent, to reduce the possibility of viral interference and thus to achieve a balanced immune response (i.e. an immune response against all the serotype contained in the composition), the amounts of each of the different dengue serotypes present in the administered vaccines may not be equal.

An “immunoeffective amount” is an amount which is capable of inducing a specific humoral immune response comprising neutralizing antibodies in the serum of a vaccinee, as evaluated by the plaque reduction neutralization test as described in section 4.1.2.2; a serum being considered to be positive for the presence of neutralizing antibodies when the neutralizing antibody titer thus determined is at least superior or equal to 1:10.

The volume of administration may vary depending on the route of administration. Subcutaneous injections may range in volume from about 0.1 ml to 1.0 ml, preferably 0.5 ml.

The optimal time for administration of the composition is about one to three months before the initial exposure to the dengue virus. The vaccines of the invention can be administered as prophylactic agents in adults or children at risk of Dengue infection. The targeted population thus encompasses persons which are naïve as well as well as non-naïve with regards to dengue virus. The vaccines of the invention can be administered in a single dose or, optionally, administration can involve the use of a priming dose followed by a booster dose that is administered, e.g. 2-6 months later, as determined to be appropriate by those of skill in the art.

The invention will be further described in view of the following figures and examples.

FIGURES

FIG. 1 is a summary of History of VDV1 pre-master seed.

FIG. 2 is a flow chart that summarises the developed manufacturing process that gives rise to the Filled Product (monovalent), “ready to use” doses.

FIG. 3 is a diagrammatic representation of VDV1 genome map. The above arrow is the polyprotein coding sequence. The below arrows represent mature peptides coding sequence. The vertical bars symbolize the nucleotidic variations between wild-type dengue 1 strain 16007 and LAV1 strain. The stars designate the nucleotidic variations between LAV1 and VDV1.

FIG. 4 shows plaque size analysis after 7 days of incubation at 37° C. for dengue-1 viruses LAV1, VDV1, and strain 16007.

FIG. 5 is a graphic analysis showing plaque size distribution for dengue-1 viruses LAV1, VDV1, and strain 16007.

FIG. 6 is a summary of Trial Design for assessment of safety of VDV1 monovalent in healthy flavivirus-naïve adults.

EXAMPLES Example 1 Sanitization

1.1 Viral RNA Purification

It was initially intended to perform sanitization of LAV1 by purifying and transfecting viral RNA directly extracted from an early seed of the vaccine strain, DEN-16007/PDK10 or DEN-16007/PDK11 (produced by Sanofi Pasteur. Titer: 4.60 logTCID₅₀/ml). Eight unsuccessful assays were carried out in that way, with RNA quantities varying from 10³ to 10⁷ copies. It was then decided to perform one adaptation passage on Vero cells, before RNA extraction and transfection.

Vero cells (VERO LS10 p142 to 145) were infected with a sample of the master seed DEN-1/PDK11, at m.o.i 0.01, and incubated at 32° C. for 5 days. Culture medium was then replaced by infection medium (containing 10 mM MgSO₄). Clear cytopathic effects were visible the following day, and presence of viral RNA in culture supernatant was confirmed by RT-PCR. Culture medium was collected at day 8 post-infection, diluted with an equivalent volume of an aqueous buffered solution comprising cryoprotective agents (pH=7.5) and kept frozen at −70° C. until use. This Vero-amplified virus was named DEN-1/V100. Its infectious titer was determined on Vero cells and was of 6.9 logTCID₅₀/ml.

The RNA purification and transfection process was performed as follows. DEN-1 V100 suspension was diluted in order to contain at least 3×10⁴ and up to 3×10⁷ TCID₅₀ or PFU of virus per milliliter. One unit of benzonase diluted in 0.01 ml of William's medium was added to 0.5 ml of virus, in order to digest DNA or RNA molecules from cellular origin, and the solution was incubated for 2 hours at 4° C. on an agitator. At the end of incubation step, 0.65 ml of a denaturing buffer containing guanidium chloride, detergent (SDS), and βmercaptoethanol (RTL-βmercaptoethanol buffer, provided in the kit RNeasy Mini kit, Qiagen Ref. 74104) were added and proteins were extracted once with phenol/chloroform (1/1) vol/vol and once with chloroform vol/vol, followed by centrifugation for 5 min at 14,000 rpm at room temperature. After each extraction, the aqueous phase was collected, taking care not to collect material (white precipitate) at the interface, and transferred to a clean 1 ml-Eppendorf tube. The RNA solution was then applied onto a QIAgen column following the recommendations of the manufacturer (RNeasy minikit, QIAgen), in order to remove traces of solvent, and eluted with 0.06 ml of nuclease-free H2O water. The presence of viral RNA was confirmed by quantitative RT-PCR, using a reference curve established with known quantities of virus, in TCID₅₀/ml.

1.2 Transfection of Vero Cells with Purified RNA

Transfection was performed using lipofectamine (LF2000 Reagent, Life Technologies), a mixture of cationic lipids that associate to RNA through charge interactions and allows transfer of the complexes into the cytoplasm of the cells by fusion with the cell membrane. The optimal quantity of LF2000 reagent was determined in a preliminary experiment by incubating Vero cells, plated 16 to 24 hours before (0.3-0.5×10⁶ cells per well in a 6 wells plate) with increasing doses (5 to 20 μl) of lipofectamine. Cells were then incubating 4 to 5 hours at 32° C., 5% CO₂, before replacing the medium by fresh culture medium without FCS, and the incubation was continued overnight at 32° C. Toxicity (round, refringent or floating cells, homogeneity of the cell monolayer) was checked regularly for 48 hours, under an inverted microscope. The highest dose of lipofectamine that was not toxic under these conditions was 10 μl and was chosen for RNA transfection.

Four transfections were carried out in parallel, using 1/10 of the purified RNA preparation (corresponding to about 4×10⁵ TCID₅₀). Twelve microliters of viral RNA solution were diluted in 500 μl of OptiMEM medium (GIBCO) containing 10 μl of LF2000 Reagent (a mixture of cationic lipids that associate to RNA through charge interactions, and allow transfer of the complexes into the cytoplasm of the cells by fusion with the cell membrane). 200 ng of yeast tRNA were added as carrier in 2 out of the 4 reactions.

The 4 transfection mixes were allowed to precipitate for 10 min at room temperature before addition to 6-wells plates of confluent Vero cells. After 4 hours of incubation at 32° C., transfection mix was removed and cells were rinsed once in PBS. Three milliliters of post-transfection medium (Williams, GIBCO) were added, and incubation was continued for 5 days at 32° C. Culture medium was then replaced by 3 ml of Dengue infection medium (Williams supplemented with 10 mM MgSO₄).

Moderate toxic effects were observed 24 hours post-transfection and disappeared on day 3. Typical cytopathic effects (round, refringent cells) were detected 6 to 8 days post-transfection in all transfection assays. Release of virus in the supernatant of these cells was confirmed by qRT-PCR. Culture fluids (3 ml) were collected at day 6 and at day 8 post-transfection, and pooled. The viruses were diluted with 6 ml of an aqueous buffered solution comprising cryoprotective agents (pH=7.5) and frozen until further amplification.

The four viral solutions such obtained after transfection were named TV (for Transfection in Vero cells) 100, TV200, TV300 and TV400, and exhibited similar infectious titers (see below):

TV100: 6.95 log TCID₅₀

TV200: 6.80 log TCID₅₀

TV300: 6.80 log TCID₅₀

TV400: 6.85 log TCID₅₀

Of note, transfection efficiency was not significantly increased in samples transfected in presence of tRNA (TV300 and TV400).

1.3 Characterization of Viruses Recovered After Transfection

Plaques sizes of DEN1-V100 and TV100 to 400 were determined. Briefly, Vero cells were plated at a density of 1.000.000 cells/cm² in culture medium containing 4% of FBS. After overnight incubation, the medium was removed and cells were infected with serial twofold or fivefold dilutions of virus. After 1.5 hour at 37° C. 5% CO₂, the inoculum was removed and cells were incubated at 37° C. 5% CO₂ in Minimal Eagle Medium (MEM) containing 1.26% methylcellulose and 10% FBS. After 11 days of incubation, plates were fixed 20 minutes in cold acetone at −20° C. and revealed by immuno-coloration with a flavivirus-specific mAb, diluted at 2.5 μg/ml. Viral plaques were measured using an image analysis software (Saisam/Microvision)

As a control, DEN-1 16007 and LAV1 were plated in parallel. The data are presented in Table 1.

TABLE 1 Plaques size of DEN-1 16007, LAV1, V100 (before transfection) and TVX00 (after transfection) Phenotype Step Virus Large Plaque Small Plaque DEN1 16007 (wt) 42 0 Master Seed/PDK11 MS-16007/PDK11 0 91 Working Seed/PDK12 WS LST-22 0 84 Bulk Seed/PDK13 D1-0010R1 0 188 VERO amplification V100 0 90 Transfected viruses TV100 0 115 TV200 1 90 TV300 0 92 TV400 0 107

The V100 virus, amplified on Vero cells, exhibits a homogeneous small plaque (SP) phenotype. Plaques are slightly larger than observed in the different samples of LAV1 (2-3 mm diameter instead of <2 mm). This SP phenotype is retained in the viruses recovered after transfection. One large plaque (LP), among 90 virus plated, was detected in TV200 sample. However, this proportion shifted to 10 LP for 82 SP plated after just one amplification passage on VERO cells suggesting that the LP population was dominant.

Of note, sequencing of attenuation-critical positions performed in parallel did not reveal any mutation in transfected viruses, compared to LAV1.

1.4 Plaque-Purifications

A sample of DEN-1/TV100 virus, presenting a homogenous SP phenotype was chosen for plaque-purification. Briefly, Vero cells were plated in 6-well plates and infected with serial dilutions of virus, in order to get between 1 and 20 plaques by plate. After 1.5 hour at 37° C. 5% CO₂, the inoculum was removed and cells were incubated under 3 ml of solid medium composed of MEM-10% FCS pre-heated at 42° C. and mixed extemporaneally with 2% of melted agarose equilibrated at 42° C. The medium was allowed to solidify at room temperature for 30 min; under flow hood, and plates were incubated in inverted position for 10 days at 32° C.-5% CO₂. A second layer of the same medium supplemented with 0.01% of neutral red was then added and plates were incubated for an additional night at 32° C. Four well-isolated small plaques were picked under sterile conditions using a micro-pipet equipped with a 0.1 ml tip, and transferred into sterile tubes containing 0.2 ml of MEM-4% FCS. The suspension was homogenized by vortexing, serially diluted in the same medium, and immediately used to infect 6-well plates of Vero cells. The protocol was repeated and a second picking of six SP was done. Each picked plaque was diluted in 1 ml of medium, before amplification on Vero cells, in T25 cm² flasks. Culture medium was collected at day 6 post-infection, diluted with the same volume of an aqueous buffered solution comprising cryoprotective agent (pH 7.5) and frozen at −70° C. All these steps were performed at 32° C.

Plaque-purified viruses were named DEN-1/TV111, DEN-1/TV112, DEN-1/TV121, DEN-1/TV131, DNE-1/TV132 and DEN-1/TV141, respectively. Infectious titers were determined on Vero cells (see below):

TV111=6.85 LogCCID₅₀/ml TV112=6.80 LogCCID₅₀/ml

TV121=6.80 LogCCID₅₀/ml TV131=6.70 LogCCID₅₀/ml

TV132=6.45 LogCCID₅₀/ml TV141=5.70 LogCCID₅₀/ml

A second amplification on Vero cells was carried out for three clones: TV111, TV112 and TV121.

1.5 Characterization of Cloned Virus

Plaques size of DEN-1/TV111, DEN-1/TV112, DEN-1/TV121, DEN-1/TV131, DEN-1 TV132 and DEN-1/TV141 candidates were determined. Spot-sequencing of specific attenuation loci was also performed and revealed no mutation (Table 2).

TABLE 2 Sequencing at attenuation-specific spots of DEN-1 viruses E NS1 NS3 NS4A NS4B NS5 Step/cell Virus 1323 1541 1543 1545 1567 1608 2363 2695 2782 5063 6048 6806 7330 9445 Non DEN-1 16007 T G A G A C A G C G A A A C attenuated/ PGMK Vaccine/PDK DEN-1 16007/ C A G A G T G C T A T G G T PDK-13 TV111 C A G A G T G C T A T G G T TV112 C A G A G T G C T A T G G T 2nd plaque- TV121 C A G A G T G C T A T G G T purification/ TV131 C A G A G T G C T A T G G T VERO TV132 C A G A G T G C T A T G G T TV141 C A G A G T G C T A T G G T Pre-master VDV1 (VERO-6) C A G A G T G C T A T G G T seed/VERO Nucleotides position are indicated below each gene and referred of DEN-1 16007 strain SEQ ID No 2.

In absence of any other criterion able to differentiate between these clones, TV121 was arbitrarily chosen as pre-master for VDV1.

In conclusion, a total number of 6 passages on VERO cells were carried out to adapt and clone the initial DEN-1 16007/PDK11 attenuated strain. Viral RNA was purified and transfected into qualified VERO cells, in conditions fitting with an industrial application (environmental control, traceability of raw material and experiments, certificate of analysis for animal-derived products). The VERO-adapted strain was cloned by plaque-purification to generate pre-master seed of VDV1 vaccine candidate, at VERO passage number 6.

Contrary to LAV1, VDV1 presents a homogenous small plaque size phenotype. Furthermore, no mutation was identified at attenuation-specific positions. Further characterizations have been performed then by determining bulk VDV1 complete sequence and phenotypic testing.

Example 2 Sequencing

The complete sequence of the virus was generated according to the following strategy. Starting from a VDV1-containing sample, the genomic RNA was extracted and purified, retro-transcribed into cDNA. Then all overlapping PCR amplifications were performed from the cDNA, with addition of the sequencing tags at both ends of each PCR product. All individual sequences were generated in automated devices and analysed. Next step consisted of the genome reconstruction by multiple alignments of all individual sequences. At this point, each unexpected nucleotide change, with regard to the reference sequence, was carefully re-analysed by going back to raw data. Such change was systematically confirmed by another sequence performed from a new PCR product. Once all ambiguities were solved, the sequenced virus genome was completed, and the new molecule was created in Vector NTi database. It can be used for inter genomes analysis, by multiple sequence alignment.

2.1 Materials

2.1.1 Viruses

The viruses to which it is referred here are DEN-1 16007; LAV-1/PDK13; VDV1, the sequences of which are given in the attached sequence listing. The complete genome sequence of these viruses is 10735 nucleotides in length.

2.1.2 Primers

All primers have been designed in Seqweb bioinformatics package (Accelrys), primer design module (Table 3).

TABLE 3 list of RT-PCT and sequencing primers Primer RT-PCR Name Primers sequences NtStart NtEnd length length Overlap D1 01+ GTTTTCCCAGTCACGACtacgtggaccgacaagaacag (SEQ ID No. 4) 12 32 38 897 −32 D1 01− AACAGCTATGACCATGggatggagttaccagcatcag (SEQ ID No. 5) 928 908 37 201 D1 02+ GTTTTCCCAGTCACGACtgaacaccgacgagacaaac (SEQ ID No. 6) 688 707 37 892 173 D1 02− AACAGCTATGACCATGaggtccaaggcagtggtaag (SEQ ID No. 7) 1598 1579 36 D1 03+ GTTTTCCCAGTCACGACttggaaatgagaccacagaac (SEQ ID No. 8) 1386 1406 38 885 146 D1 03− AACAGCTATGACCATGgaaacaccgctgaacaaaac (SEQ ID No. 9) 2289 2270 36 D1 04+ GTTTTCCCAGTCACGACggttcaagaagggaagcag (SEQ ID No. 10) 2106 2124 36 903 183 D1 04− AACAGCTATGACCATGttctatccagtaccccatgtc (SEQ ID No. 11) 3028 3008 37 D1 05+ GTTTTCCCAGTCACGACcagaataccaccttcatcatcg (SEQ ID No. 12) 2804 2825 39 868 232 D1 05− AACAGCTATGACCATGttcccatccccatcttgtc (SEQ ID No. 13) 3689 3671 35 D1 06+ GTTTTCCCAGTCACGACggaaatcagaccagtcaaggag (SEQ ID No. 14) 3418 3439 39 913 233 D1 06− AACAGCTATGACCATGtgttgtgtgaggcaccagag (SEQ ID No. 15) 4349 4330 36 D1 07+ GTTTTCCCAGTCACGACgcaaaccactaaccatgtttc (SEQ ID No. 16) 4077 4097 38 901 259 D1 07− AACAGCTATGACCATGccacttgttgtcaccactc (SEQ ID No. 17) 4995 4977 35 D1 08+ GTTTTCCCAGTCACGACccaagggaagagactggaac (SEQ ID No. 18) 4699 4718 37 908 210 D1 08− AACAGCTATGACCATGtcctgatttgatgcttggaac (SEQ ID No. 19) 5626 5606 37 D1 09+ GTTTTCCCAGTCACGACaagcacattttaccgatccag (SEQ ID No. 20) 5376 5396 38 900 182 D1 09− AACAGCTATGACCATGgtcgtagtttctttctttctccttc (SEQ ID No. 21) 6299 6275 41 D1 10+ GTTTTCCCAGTCACGACgcaatagacggggaatacag (SEQ ID No. 22) 6074 6093 37 809 138 D1 10− AACAGCTATGACCATGatgatggtggttttcagcag (SEQ ID No. 23) 6901 6882 36 D1 11+ GTTTTCCCAGTCACGACgtgttgcttattccagagcc (SEQ ID No. 24) 6725 6744 37 877 228 D1 11− AACAGCTATGACCATGgctgtcttttccatttttctcc (SEQ ID No. 25) 7622 7601 38 D1 12+ GTTTTCCCAGTCACGACactttgcacatcacagatcc (SEQ ID No. 26) 7354 7373 37 821 175 D1 12− AACAGCTATGACCATGttcgcactagcattcctcc (SEQ ID No. 27) 8192 8174 35 D1 13+ GTTTTCCCAGTCACGACcacctgagaaatgtgacacc (SEQ ID No. 28) 7980 7999 37 907 207 D1 13− AACAGCTATGACCATGtttccttgtttatgaagctccc (SEQ ID No. 29) 8907 8886 38 D1 14+ GTTTTCCCAGTCACGACcaaaagcgaaacgaggcac (SEQ ID No. 30) 8661 8679 36 894 221 D1 14− AACAGCTATGACCATGgtttcaccacacagtcatctcc (SEQ ID No. 31) 9575 9554 38 D1 15+ GTTTTCCCAGTCACGACagaccagcgaaaaatggaac (SEQ ID No. 32) 9314 9333 37 865 262 D1 15− AACAGCTATGACCATGtcccaatgagccttctcac (SEQ ID No. 33) 10196 10178 35 D1 16+ GTTTTCCCAGTCACGACgctaatgctatctgttcagcc (SEQ ID No. 34) 9896 9916 38 812 D1 16− AACAGCTATGACCATGtgattcaacagcaccattcc (SEQ ID No. 35) 10726 10707 36 −28 D1 16i+ ccatggaagctgtacgc (SEQ ID No. 36) 10480 10496 17 D1 16i− gagacagcaggatctctgg (SEQ ID No. 37) 10671 10652 19

2.2 Methods

2.2.1 Viral RNA Purification

From previous experience, a minimal of 1000 DICC₅₀ is required to get a positive RT-PCR reaction in the next steps. This means that a minimum virus titer of 10⁴ DICC₅₀/mL is necessary. Virus genomic RNA was purified using QIAamp viral RNA mini kit (Qiagen), according to the manufacturer's recommendations. Briefly, a volume of 140 μl from a crude viral sample was incubated in the presence of the lysis solution, and loaded onto a kit column. After washing steps, the purified viral RNA was eluted by 60 μl of sterile nuclease-free water containing 1 μl (40 units) of RNAse inhibitor (RNAse Out, Sigma).

2.2.2 Reverse Transcription

Viral RNA was reverse transcribed into cDNA by a reverse transcriptase (reverse iT) from ABGene. Again, standard operating conditions were applied, using 10 μl of purified RNA, in a final reaction volume of 20 μl. The reaction was initiated by hybridization of the minus strand primers. One RT reaction per PCR was performed (Table 1). The cDNA synthesis was obtained by 45 min incubation at 47° C.

2.2.3 PCR

All PCR were performed with Expand High Fidelity PCR system (Roche diagnostics), using all 16 pairs of primers (+) and (−) from Table 1. PCR conditions were the following ones:

RT 2 μl 10 × buffer 2.5 μl dNTP mix (10 mM) 2 μl Primers 0.8 μl each H2O 16.4 μl Enzyme 0.5 μl PCR program Denaturation 94° C.  2 min Denaturation 94° C. 15 sec Hybridization 55° C. 30 sec {close oversize brace} 40 cycles Elongation 68° C.  1 min Elongation 68° C.  5 min

All PCR products were controlled by electrophoresis on agarose gel.

2.2.4 Sequencing

The major part of the sequence reactions has been outsourced to Genome Express. Genome extremities, ambiguities, some inter-PCR junctions, and regions not sequenced by Genome Express for technical reasons were performed in-house.

-   -   Sequencing at Genome Express: PCR products were shipped at +4°         C., and sequencing results were received as informatic sequence         files. Text file, quality files and chromatograms are available         for each individual sequence. After sequence alignment, all         discrepancies were checked on the chromatogram, and corrected if         identified as sequence algorithm errors.     -   In-house sequencing: Sequence reactions were performed on         thermocycler PTC-200 (MJ Research), with Sequitherm Excell II LC         kit (Epicentre). Each PCR product was sequenced on both strands         independently in a single reaction. Reactions were loaded onto a         sequence electrophoresis gel. Run and analysis of sequence were         performed on the automated sequencer Gene ReadIR 4200 (Li-Cor).

DNA up to 200/250 ng Reaction buffer 7.2 μl Primers (1-2 pM) 1.5 μl each Enzyme 1 μl H2O up to 20 μl PCR program Denaturation 92° C.  2 min Denaturation 92° C. 15 sec Hybridization 50° C. 30 sec {close oversize brace} 30 cycles Elongation 70° C.  1 min Elongation 70° C. 10 sec

Denaturation of samples 3 min at 95° C. and ice cooling just before samples loading.

Electrophoresis parameters Gel parameters Voltage 1500 V Gel hight 41 cm Current 35 mA Gel thickness 0.2 mm Power 40 W Temperature 45° C. Run time 9H00 Scan speed 3

2.3 Results

All PCR fragments were sequenced from both ends using a common PCR added tail, i.e. a specific motif which has been added at 5′ end of all primers:

5′ primers: M13SEQ-GTTTTCCCAGTCACGAC (SEQ ID No. 38) 3′ primers: M13REV-AACAGCTATGACCATG (SEQ ID No. 39)

M13-SEQ and -REV sequences correspond to universal M13 primers motifs (New England Biolabs references).

For final contig assembly, a quick analysis was performed in Vector NTi, in ContigExpress module (Informax). The LAV1 reference sequence was compared with all individual sequencing results. In such conditions, all results could be aligned at the right place on the complete genome, even when some regions were still missing contig assembly, giving a quick visualization of the overall genome alignment.

2.3.1 Complete VDV1 Sequence Assembly

The final sequence alignment was performed in Vector NTi, AlignX module (Informax). The classical multiple sequence alignment algorithm ClustalW (Thompson et al., 1994) was used by the software to build the global alignment. All the sequence results were aligned together with the LAV1 reference sequence, thus allowing for a better reconstruction of the genome. Any discrepancy in the sequence with regard to the reference required a confirmation on another independent sequence reaction. The complete sequence of VDV1 is shown in SEQ ID No.1.

Some ambiguities are often found in single sequences, especially near sequence extremities. This is inherent to the somewhat poor quality of the reaction at both ends of any PCR fragment. Such poor quality sequences were excluded from the alignment, until two other independent sequence reactions were available from other PCR products. Discrepancy towards the reference was not taken into account in the final alignment when not confirmed in at least two independent other PCR sequences matching the consensus. Conversely, any discrepancy confirmed on two independent sequences was kept in the final sequence.

Table 4 summarizes the characteristics of each individual sequence reaction, indicating start, end and length. Overlaps between adjacent PCR are also indicated, as well as differences with regard to the reference sequence in the last column.

TABLE 4 Dengue VDV1 individual sequences characteristics Name Start End Size Overlap Comments D1 01+ 35 921 886 0 D1 01− 899 33 866 209 D1 02+ 712 1596 884 D1 02− 1569 713 856 196 D1 03+ 1415 2277 862 D1 03− 2253 1400 853 160 D1 04+ 2133 3027 894 2719 G > A (NS1-100 s) D1 04− 3000 2117 883 212 2719 G > A (NS1-100 s) D1 05+ 2834 3681 847 D1 05− 3654 2815 839 247 D1 06+ 3451 4332 881 D1 06− 4325 3434 891 243 D1 07+ 4113 4987 874 D1 07− 4961 4089 872 245 D1 08+ 4742 5564 822 D1 08− 5583 4742 841 183 D1 09+ 5400 5916 516 D1 09− 6274 5758 516 197 5962 C > A (NS3-481 N > K); 2 sequences D1 10+ 6114 6868 754 D1 10− 6883 6077 786 135 D1 11+ 6761 7504 743 D1 11− 7597 6733 864 217 D1 12+ 7381 8034 653 7947 A > G (NS5-125 K > R) D1 12− 8143 7380 763 141 7947 A > G (NS5-125 K > R) D1 13+ 8003 9730 727 D1 13− 8857 8002 855 182 D1 14+ 8687 9472 785 D1 14− 9544 8675 869 200 D1 15+ 9344 10170 826 D1 15− 10162 9399 763 253 D1 16+ 9917 10261 344 2 sequences D1 16− 10706 10394 312 0 D1 16i+ 10500 10706 206 D1 16i− 10649 10204 455

The two extremities of the genome could not be sequenced from PCR amplification, because cDNA synthesis and PCR DNA reaction required oligonucleotides complementary to the ends of the genome. During the amplification step, these oligonucleotides are incorporated into the PCR fragment. The sequence result is that of the synthetic oligonucleotide, and not that of the virus itself. PCR from both ends of the virus genome did work properly, suggesting that the viral sequence was not significantly different from the oligonucleotide sequence (if it had been the case, PCR amplification should have failed or at least should have been of poor quality). The two extremities of the genome could not be distinguished from all other PCR amplifications. So, in the reconstructed genome (SEQ ID No.1), both genome ends were considered as identical to oligonucleotide sequences (and also identical to the reference). At 5′ end, the sequence is that of nucleotides 1 to 34. At 3′ end, the sequence is that of nucleotides 10707 to 10735.

2.3.2 Sequence Comparison

Direct sequence comparison between VDV1 strain and LAV1 reference shows a series of 3 nucleotides differences. Table 5 gives the complete list of these positions.

TABLE 5 Sequence comparison between LAV1 and VDV1 strains Nucleotides Amino Acids Nt AA Position LAV1 VDV1 Region Position LAV1 VDV1 Notes 2719 G A NS1 100 G G Silent 5962 C A NS3 481 N K 7947 A G NS5 125 K R

Nucleotide change in position 2719 is silent at the amino acid level. The second difference in position 5962 triggers an amino acid change at NS3-481 (asparagine to lysine). Both are hydrophilic, but lysine is positively charged, whereas asparagine is not. The last difference is located in NS5 peptide, substituting lysine to arginine in position NS5-125. Such amino acid substitution is relatively conservative from a chemical point of view, both arginine and lysine residues being hydrophilic and positively charged.

TABLE 6 Search of discrepancies on other Dengue 1 strains Nucleotide position Number of strains sharing on VDV1 strain the same nucleotide 2719 24/40  5962 6/40 7947 1/40

When performing sequence alignment between all available Genbank serotype 1 Dengue genomic sequences, it appears that most of the identified differences are also present on other strains (see Table 6). One position is unique in the VDV1 strain (position 7947; NS5-125).

Thus, the full genomic sequence of a VDV1 strain of the dengue virus has been established.

Three nucleotide differences have been detected with regard to the parent LAV1 genomic sequence. VDV1 vaccine strain is derived from LAV1, through virus “sanitization” and passage from dog to monkey cells.

Differences between LAV1 and VDV1 can have several origins. First, cloning steps can elect a viral subpopulation that is not 100% identical to the major sequence previously detected in LAV1. Second, LAV1 has been produced on PDK cells, whereas VDV1 has been made on Vero cells. Such passage from dog to monkey cells potentially induces virus changes that reflect adaptation to the new cell line. Third, as for all RNA viruses, the lower viral RNA polymerase fidelity triggers a higher genomic mutation rate than DNA polymerases do.

In term of sequences only 3 differences between LAV1 and VDV1 were observed, corresponding to only 2 amino acids substitutions. All 14 nucleotide positions that have been linked to LAV1 viral attenuation are conserved in VDV1. Furthermore the sequences of master and bulk VDV1 have been compared (Table 7).

TABLE 7 Dengue 1 nucleotide differences between wild type 16007 strain and attenuated LAV1/PDK13 and VDV1 strains 1323/ 1541-3/ 1545 1567/ 1608/ 2363/ 2695/ 2719/ Virus E-130 E-203 E-204 E-211 E-225 E-477 NS1-92 NS1-100 DEN-1 T GaA G A C A T G 16007 Val Glu Arg Gln Ser Met Asp Gly LAV1/ C AaG A G T G C G PDK13 Ala Lys Lys Gln Leu Val Asp Gly VDV1 C AaG A G T G C A Master Ala Lys Lys Gln Leu Val Asp Gly VDV1 C AaG A G T G C A Bulk Ala Lys Lys Gln Leu Val Asp Gly 2782/ 5063/ 5962/ 6048/ 6806/ 7330/ 7947/ 9445/ Virus NS1-121 NS3-182 NS3-481 NS3-510 NS4a-144 NS4b-168 NS5-125 NS5-624 DEN-1 C G C A A A A C 16007 Ala Glu Asn Tyr Met Gln Lys Ser LAV1/ T A C T G G A T PDK13 Ala Lys Asn Tyr Val Gln Lys Ser VDV1 T A A T G G G T Master Ala Lys Lys Tyr Val Gln Arg Ser VDV1 T A A T G G G T Bulk Ala Lys Lys Tyr Val Gln Arg Ser Complete VDV1 master seed sequence was aligned with the bulk sequence. No difference between the two sequences was observed, indicating genetic stability across passages. VDV1 shows a remarkable genetic stability with regard to its LAV1 parent.

Example 3 Characterization

The objective of these studies was to assess whether changes in attenuation markers occurred through passages.

The flow chart shown on FIG. 2 summarises the developed manufacturing process that gives rise to the Filled Product (monovalent), “ready to use” doses

Briefly, after 2 successive passages on Vero cells of the Viral Pre-Master Seeds delivered by the Research department, the respective working seeds were obtained. The final virus cultivations were also conducted by infection of a Vero cell suspension. The viruses produced are then harvested. DesoxyRiboNucleic Acid (DNA) was digested according to an enzymatic treatment. Impurities were removed by ultrafiltration. Infectious titers were enhanced by a concentration step. An aqueous buffered solution comprising cryoprotective agents (pH=7.5) is added and this 0.22-μm filtrated mixture is then diluted at the targeted dose within the same solution. The active substance is then filled into glass vials, freeze-dried, and stored before use.

3.1 Phenotypic Markers

The results are shown in Table 8. The validated tests performed for the master seed and the bulk are:

Plaque size: the assay was performed in Vero cells at 37° C. after 7 days of incubation. Sizing of the plaques was performed by Saisam v.5.2.0 (Microvision Instruments) dedicated software, after image capture with a video camera. Two populations (0.3 mm and 0.8 mm) were detected in LAV1. The major population was the smallest. After adaptation to Vero cells and biological cloning, VDV1 plaque size distribution appears homogenous, with more than 98% of the population showing a single peak, centered to 0.8 mm in diameter. These plaques are clearly distinct from plaques obtained with DEN-1 16007 virus (see FIGS. 4 and 5).

Temperature sensitivity: monovalent 1 exhibits clear restricted growth at 39° C. with respect to the non-temperature sensitive (Ts), wild-type (WT) D1-16007. This was demonstrated both by infectious titer assay and by viral RNA quantification. Master, bulk and passage 18 (10 passages after the bulk passage) of the monovalent 1 seed display 90% or more of titer reduction at 39° C., compared to 37° C.

TABLE 8 Summary of DEN-1 viral phenotypes Neurovirulence in Temperature sensitivity newborn (Percent titer reduction at 39° C.)_(Fold-reduction) Swiss Webster mice Virus Score Day 3 Day 4 Day 5 Day 6 Mortality_(n)* AST (S.D.) D1-16007 − 62.1_(2.6) 59.3_(2.5) 56.3_(2.3) (−28.5_(−1.4)) 6.25%₁₆ 19.0 (0.0) D1-PDK13 + 87.1_(7.8) 91.3_(11.5) 95.5_(22.2) 96.5_(28.6) 0.00%₁₆ n.a. VDV1 MS + 97.2_(35.7) 97.7_(43.5) 98.8_(83.3) 99.5_(200.0) 0.00%₁₆ n.a. *n: number of animals

3.2 Genotypic Markers

VDV1 vaccine strain can be distinguished from parental strains at the genomic level. Attenuation-specific loci have been identified. These loci are conserved in master and bulk seeds.

Example 4 Immunogenicity, Viremia, and Toxicology in Monkeys

The most solid and numerous data that can be obtained in monkeys concern immunogenicity and viremia. Viremia, in particular, has been identified as one of the factors associated with virulence and disease severity in humans, and then constitutes an important parameter to consider. Obviously, immunogenicity is a key parameter when testing vaccines.

Inventors have established minimal/maximal values for viremia and immunogenicity.

TABLE 9 Minimal requirements for responses induced by Dengue vaccine candidates in monkeys, as measured in Vero or LLC-MK2 cells by plaque assay (these cells being considered equivalent in such an assay) Viremia mean duration Viremia mean peak titer Mean neutralizing titer (days) (log 10 pfu) Day 30 (all serotypes being (all serotypes being (for each serotype) considered) considered) PRNT 50 ≦3 days ≦1.5-2 ≧80 pfu: plaque forming unit PRNT 50: Plaque Reduction Neutralization Titer 50 (titre corresponding to a reduction of 50% of plaque number)

4.1 Material and Methods

4.1.1 Monkey Experiments

Monkey experiments were carried out according to European guidelines regarding animal experiments.

Immunizations were performed on cynomolgus monkeys (Macaca fascicularis) originating from Mauritius (CRP Le Vallon). Monkeys were quarantined for 6 weeks in the animal facility of Sanofi Pasteur before immunization.

Monkeys were immunized by subcutaneous (SC) route in the arm with vaccines in a volume of 0.5 ml (see each respective section). After light anesthesia with ketamine (Imalgene, Merial), blood was collected by puncture of the inguinal or saphene veins. At days 0 and 28, 5 ml of blood were sampled for evaluating antibody responses while between days 2 and 10 only 1 ml of blood was sampled for evaluating viremia. Blood was collected on ice and kept on ice until serum separation. To do so, blood was centrifuged for 20 minutes at 4° C., and serum collected and stored at −80° C. until testing in Rich Kinney's laboratory. Shipment to USA was performed in dry ice.

4.1.2 Viremia and Neutralizing Antibody Responses (Plaque Reduction Neutralization Test, PRNT)

All analyses were performed in the laboratory of R. Kinney in CDC, Fort Collins, USA. Serum samples were shipped and stored at −80° C. until the time of testing. At the time of first thawing, the samples were tested for viremia, and a 1:5 dilution of the serum was made. The 1:5 serum dilutions were inactivated for 30 min at 56° C. before testing for neutralizing antibodies.

4.1.2.1 Viremia

0.125 ml of serum was added to 0.125 ml of diluent (RPMI medium) in the first well of 96-well plate and serial 10-fold dilution series were done, transferring 0.025 ml into 0.225 ml of diluent for each dilution. 0.2 ml of 10^(0.3)-10^(5.3) dilution series was plated in 6-well plate of Vero cells (virus was adsorbed at 37° C. for 1.5 hour, overlayed with 4 ml of agarose lacking neutral red, overlayed 6-7 days later with 2 ml of agarose containing neutral red, and plaques counted). The limit of virus detection was =10 PFU/ml. For controls stock DEN-1 6007 PDK-13 (LAV1) vaccine was plated.

4.1.2.2 PRNT (Plaque Reduction Neutralization Test)

Neutralizing antibodies were quantified as described in Huang et al. (2000). Briefly, 0.2 ml of heat-inactivated, 1:5 dilution of serum was added to the first well of 96-well plate and serial 2-fold dilution series were made, transferring 0.1 ml into 0.1 ml of diluent (RPMI medium) for each dilution. This resulted in a 1:10-1:320 serum dilution series. 0.1 ml of DEN virus (60-160 PFU; parental DEN1 16007 virus) was added to each serum dilution well for a total of 0.2 ml of serum-virus mixture. 96-well plates were incubated overnight at 4° C. 0.1 ml of serum-virus mixtures (containing 30-80 PFU of input virus) were plated in 6-well Vero plates (as indicated above in the Viremia section) and plaques were counted after staining with neutral red. Multiple back titrations of the input viruses at 2-fold, 1-fold, and 0.5-fold test concentrations provided direct experimental determination of the input PFU, which was the basis for determining 50% (PRNT₅₀) and 70% (PRNT₇₀) endpoint antibody titers. A negative serum result should have a neutralizing antibody titer of <1:10. Sera showing neutralization titers of 320 were retested at dilutions 1:80-1:2560 for determination of endpoint titer.

4.2 Evaluation of VDV Candidates

4.2.1 VDV 1/Pre-master

Purification/selection of D1 candidate has been conducted as described in example 1. The selected clones (based on phenotypic markers and sequence) have been tested in sanofi pasteur as described in Material and Methods (Marcy l'Etoile animal facility, I15) on male cynomolgus macaques (Macaca fascicularis, mean weight 3.1 kg) originating from CRP Le Vallon, Mauritius.

After immunization on D0, viremia was followed from D2 to D10, and immunogenicity measured at D0 and D28. All viruses and vaccines, when in liquid form, were kept at −70° C.

LAV1: titre: 10^(3.9) DICC₅₀/ml; lyophilized, resuspended in 0.5 ml of PBS (containing Ca²⁺ and Mg²⁺; CaCl₂.2H₂O 0.133 g/l; MgCl₂.6H₂O, 0.1 g/l) and administered in totality.

Premaster VDV1 DEN1-TV111: Titre: 10^(5.9) DICC₅₀/ml; liquid, diluted at 10^(5.3) pfu/ml in PBS (containing Ca²⁺ and Mg²⁺; CaCl₂.2H₂O 0,133 g/l; MgCl₂.6H₂0, 0.1 g/l); 0.5 ml administered.

Injection was done by SC route in the arm with a 23G1 needle, at a 10⁵ DICC₅₀ dose for VDV1.

The results are as presented in Table 10. Titrations at day 28 were carried out in triplicate (PRNT₇₀) or in duplicate (PRNT₅₀).

TABLE 10 VDV1 PreMaster immunogenicity AvP monovalent VDV1 (Exp A) DEN Monkey study (F. Mi.DEN003.Si): PRNT and Viremia Results Neutralizing Antibody Titer Viremia (PFU/ml in Vero cells) Day (−15) Day 28 Day Day Day Day Serum Group PRNT₇₀ PRNT₅₀ PRNT₇₀ PRNT₅₀ −15 2 3 Day 4 Day 5 Day 6 Day 7 Day 8 Day 9 10 AD LAV <10/<10 <10 320/160/160 160/320 0 0 0 0 0 0 0 0 0 0 333 DEN-1 AC <10/<10 <10 640/640/640 1280/1280 0 0 0 0 0 0 0 0 50 150 763 AD <10/<10 <10 320/160/160 160/320 0 0 0 0 0 0 0 0 0 0 209 AC <10/<10 <10 160/80/160 160/160 0 0 0 0 0 0 50 0 0 0 755 AC VDV <10/<10 <10 160/80/80 160/160 0 0 0 0 0 0 0 0 0 0 775 DEN-1 AC TV111 <10/<10 <10  20/10/10 20/20 0 0 0 0 0 0 0 0 0 0 881 AD <10/<10 <10 320/80/80 160/80  0 0 0 0 0 0 0 0 0 0 145 AD <10/<10 <10  20/10/10 20/20 0 0 0 0 0 0 0 0 0 0 113 Virus Exp#1 Exp#2 Exp#3 DEN-1 112PFU 45PFU 101PFU

Briefly, responses were rather homogeneous within each group, and some clear tendencies could be identified for each construct. No dramatic differences were found between VDV1 and LAV1: low and late viremia was observed in some LAV1 monkeys. VDV1 looked satisfactory, and in particular presented no viremia.

4.2.2 VDV 1 Bulk

As immunogenicity of the vaccines had been tested at the Premaster stage, a further experiment was designed to test each monovalent at the Bulk stage.

Male Macaca fascicularis monkeys were used as before, originating from C.R.P. Le Vallon, Ile Maurice (24 monkeys, mean weight 3.4 kg).

VDV1; Batch: Titre: 8.37 log 10 DICC₅₀/ml

Placebo: PBS with Ca²⁺ and Mg²⁺

Vaccines were diluted at 10^(5.3) DICC₅₀/ml in PBS (containing Ca²⁺ and Mg²⁺; CaCl₂.2H₂O 0.133 g/l; MgCl₂.6H₂O, 0.1 g/l); 0.5 ml administered by SC route in the arm with a 23G1 needle, corresponding to a dose of 10⁵ DICC₅₀.

Viremia and immunogenicity have been measured as usual in CDC by R Kinney. The results are shown in Table 11.

VDV1 monovalent vaccine induced a significant immune response while viremia was absent. Thus, this monovalent VDV1 fulfilled the success criteria initially defined in monkeys.

TABLE 11 VDV Bulk VDV1 immunogenicity and viremia Monkey study (F.MI.DEN004.Mk): Monovalent and Tetravalent VDV1 Neutralizing Antibody Titer Viremia (PFU/ml in Vero cells) Day (−14) Day 28 Day Day Day Day Day Day Monkey Group PRNT₅₀ PRNT₇₀ PRNT₅₀ PRNT₅₀ −14 2 3 4 Day 5 6 Day 7 8 Day 9 AE 484 VDV DEN-1 — — 14 5 0 0 0 0 0 0 0 0 0 AE 627 — — 8122 4558 0 0 0 0 5 0 0 0 0 AF 115 — — 359 202 0 0 0 0 0 0 0 0 0 AF 227 — — 557 367 0 5 0 5 0 0 0 0 0 Geo Homologous — — 388 203 Mean response AE 538 Placebo —/—/—/— —/—/—/— 2.5/—/2/2 —/—/—/— 0 0 0 0 0 0 0 0 0 AE 548 —/—/—/— —/—/—/— —/—/1/2 —/—/—/— 0 0 0 0 0 0 0 0 0 AE 556 —/—/1.5/2 —/—/—/— 1/—/—/— —/—/—/— 0 0 0 0 0 0 0 0 0 AE 572 —/—/1.5/5 —/—/1.5/2 5/—/—/2 —/—/—/— 0 0 0 0 0 0 0 0 0 Geo Response —/—/1.2/3 —/—/1/1 2/—/ —/—/—/— Mean against the 1.2/1.6 four serotypes D1/D2/ D1/D2/ D1/D2/ D1/D2/ D3/D4 D3/D4 D3/D4 D3/D4

4.3 Neurovirulence Tests in Monkeys

For each virus type, 10 cynomolgus monkeys from Mauritius were inoculated with VDV1 master seed by the intracerebral route (10^(7.23) CCID₅₀/mL in the thalamus of each hemisphere). At the end of the test, the monkeys were sacrificed and perfused with formalin solution. Tissue samples were taken from the brain of each monkey (medulla oblongata, pons and cerebellum, midbrain, thalamus including the left and the right parts, the left and the right of the cerebral cortex). Sections were cut at a thickness of 8 μm and stained by eosin and gallocyanin.

No histopathological signs of pathogenicity were observed in the monkey brains injected with VDV1 seeds.

Example 5 Safety of Monovalent VDV1 in Healthy, Flavivirus-Naive Adults Aged 18 to 40 Years

The aim of this phase 1 trial is to document the safety, viremia, and immunogenicity profiles of monovalent VDV1 at a virus concentration of 10⁴ CCID₅₀ compared to Stamaril® (used as control group) in flavivirus-naive adults. Single injections are given, with follow-up at 6 and 12 months. For safety precaution, sequential inclusions are performed in the study.

Enrollment and vaccinations are therefore staggered; a 1st cohort (n=4 per group, total n=12) have been vaccinated. The safety data collected up to Day 28 have been reviewed by an Independent Data Monitoring Committee (IDMC) and by the Royal Adelaide Hospital Investigational Drugs Subcommittee (IDSC) before deciding to proceed with the vaccination of the remaining subjects (n=8 per group, total n=16). A schematic representation of the trial design is provided in FIG. 6.

After administration of the vaccine the patient are regularly submitted to various clinical examination and testing. A summary of this follow up is given in table 12 below.

The enrolled population consists of adults aged 18 to 40 years (i.e. the day of the 18th birthday to the day before the 41st birthday) on day of inclusion who are flaviviruses-naïve [persons presenting vaccination against flavivirus diseases (e.g. yellow fever, Japanese encephalitis, dengue fever); or history of flavivirus infection (confirmed either clinically, serologically or microbiologically) or previous residence in or travel to areas with high dengue infection endemicity (whatever the duration), or residence in or travel to North Queensland for 2 weeks or more) were excluded]

TABLE 12 Flow chart for follow up Visit Number V01 V02 V03 V04 V05 V06 V07 V08 V09 V10 V11 V12 Trial timelines

D 0 D 2 D 4 D 6 D 8 D 10 D 12 D 14 D 16 D 28 D 180 D 365 Time Windows ±1 d ±1 d ±4 d ±15 d ±30 d Clinical Examination √ √ √ √ √ √ √ √ √ √ √ √ Vital signs (BP, pulse √ rate) Oral temperature √ √ √ √ √ √ √ √ √ √ Blood Sampling: Serology HBV/HCV/HIV √ √ √ √ √ √ Biological Safety √ √ √ √ √ √ √ √ √ Viremia √ √ √ √ √ Immunogenicity √ √ √ √ √ √ √ √ √ √ Cytokines in serum √ √ PBMCs for T cell √ √ (subset) immediate surveillance √ Local & systemic events √ √ √ √ √ √ √ √ √ √ √ √ V: Visit D: day

Time intervals between visits will be calculated from the date of study vaccination which might differ from the date of visit (e.g. in case a temporary exclusion criterion is met). V06 and V07 must be done with at least 1-day interval.

The products tested are:

The vaccine evaluated is a lyophilised product in a vial that is reconstituted extemporaneously with the diluent provided separately:

Active ingredient: 4±0.5 log₁₀ CCID₅₀ of either monovalent Vero dengue virus serotype 1 (VDV1) per 0.5 mL dose;

Diluent: Sterile NaCl 4% solution for vaccine reconstitution.

The reconstituted vaccine, i.e 0.5 mL of NaCl 4% solution of monovalent VDV1, should be used immediately or be maintained until use +2° C. and +8° C.

The 0.5 mL vaccine dose is administered subcutaneously in the deltoid region.

The control vaccine Stamaril®, is a yellow fever vaccine produced by Aventis Pasteur. Stamaril® is presented as a lyophilised, avian-leukosis-free, stabilised product to be reconstituted with a diluent immediately before use. (Active ingredient: Live attenuated yellow fever virus (17D strain): ≧1,000 mouse Lethal Dose 50% (LD₅₀)/Diluent: Sterile NaCl 4% solution).

The control vaccine is administered subcutaneously in the deltoid region.

The preliminary results of the trial are reported in Table 13 below.

TABLE 13 preliminary safety data Day Day 0 Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8 Day 9 Day 10 11 Day 12 Day 13 LOCAL SOLICITED Pain 1 Erythema Induration 1 Edema LOCAL UNSOLICITED Bruise 1 1 Pruritis OTHER SOLICITED Temp ≧37.5 C. 1 1 Rigors 1 1 1 Malaise 2 1 1 1 1 1 1 1 2 1 2 Asthenia 1 2 1 1 2 1 1 2 1 1 Anorexia 1 1 1 1 1 1 Nausea 1 2 1 1 1 1 1 1 2 Vomiting 1 1 1 1 1 1 Stomach Pain 1 1 2 2 1 1 1 1 1 1 1 Headache 2 1 2 2 1 1 1 2 1 1 1 1 Myalgia 1 1 1 1 1 2 Arthralgia 1 1 1 1 1 1 1 Avoidance of 1 1 1 light Conjunctivitis Eye Pain 1 1 RASH Macular 1 (1%) Papular 1 (1%) Maculo- 1 1 papular (90%) (90%) OTHER UNSOLICITED Decreased 2 1 WCC Neutropenia 2 1 Increased 1 1 aPPT Elevated CK 1 Odd dreams 1 Low abdo pain 1 1 1 (kidneys/liver) Diarrhoea 1 1 1 1 1 1 1 1 1 Sore throat 1 1 Cough 1 Early 1 menstruation Tiredness 1

Table 13 shows that biological abnormalities (WCC reductions, platelet count reductions) have all been mild. The symptoms have been mainly malaise, nausea, diarrhea and occasional vomiting. They have been of moderate severity. One significant rash—typical “viral” maculopapular rash, onset day 12, 90% coverage.

The safety data of the second cohort are also satisfactory with no biological abnormality recorded. All subjects have antibody response 28 days after vaccination against dengue 1 (titer between 1315 and 13150).

REFERENCES

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1. A live attenuated dengue-1 virus strain which comprises sequence SEQ ID NO: 3 wherein at least nucleotides at positions 5962 and 7947 are mutated, with the proviso that the following nucleotides are not mutated: 1323, 1541, 1543, 1545, 1567, 1608, 2363, 2695, 2782, 5063, 6048, 6806, 7330, and
 9445. 2. The dengue-1 virus strain according to claim 1, wherein the nucleotide at position 2719 is further mutated.
 3. The dengue-1 virus strain according to claim 1, wherein SEQ ID NO: 3 comprises the mutations 2719 G>A, 5962 C>A, and 7947 A>G.
 4. The dengue-1 virus strain according to claim 1, which further comprise a substitution of one or more nucleotides in a given codon position which results in no alteration in the amino acid encoded at that position.
 5. The dengue-1 virus strain according to claim 1, which comprises SEQ ID NO:
 1. 6. An immunogenic composition comprising a live attenuated dengue-1 virus strain which comprises sequence SEQ ID NO: 3 wherein at least nucleotides at positions 5962 and 7947 are mutated, with the proviso that the following nucleotides are not mutated: 1323, 1541, 1543, 1545, 1567, 1608, 2363, 2695, 2782, 5063, 6048, 6806, 7330, and 9445, in a pharmaceutically acceptable carrier.
 7. The immunogenic composition according to claim 6, wherein said live attenuated dengue-1 virus strain comprises SEQ ID NO: 3 in which A at position 2719 is substituted for G, A at position 5962 is substituted for C, and G at position 7947 is substituted for A.
 8. The immunogenic composition according to claim 6, wherein said live attenuated dengue-1 virus strain further comprises a substitution of one or more nucleotides in a given codon position which results in no alteration in the amino acid encoded at that position.
 9. The immunogenic composition according to claim 6, wherein said live attenuated dengue-1 virus strain comprises SEQ ID NO:
 1. 10. The immunogenic composition according to claim 6, which further comprises at least one live attenuated dengue virus selected from the group consisting of serotype 2, serotype 3, and serotype
 4. 